The use of Petri dishes and contact plates for growing colonies of microorganisms such as bacteria or fungi is well known. A Petri dish typically comprises an open dish for holding microorganism growth medium and an overlapping cover that isolates the growth medium and microorganisms from the external environment. A contact plate is a much smaller version of a Petri dish, the dish component of which is pre-loaded with growth medium and is provided with a base for grasping the contact plate so as to permit it to be pressed against a surface to obtain a sample of any microorganism(s) present on the sampled surface.
Petri dish covers may be loosely fitting so that the seal on the dish arises simply from the weight of the cover bearing upon the cylindrical side walls of the dish. Petri dish covers may also be tightly securable to and detachable from the dish, which prevents opening of the Petri dish when it is accidentally bumped or knocked over. One such Petri dish design is disclosed in U.S. Pat. No. 3,769,936, wherein the cover may be secured to the dish by ribs in the side walls of the cover that resiliently contact the side walls of the dish so as to form a compression fit. However, this design has the inherent drawback that the compression fit is often either too tight to allow ready disengagement between the cover and dish or too loose, which can lead to accidental spillage or contamination when handling the Petri dish.
Contact plates are typically fabricated from polymeric material in mass quantities at a sufficiently low cost as to be disposable after a single use. The dish portion of the contact plate is filled with a generally convex mound of growth medium. With the lid removed, the contact plate is grasped by its base and the mound of growth medium is pressed against a surface to be tested for bacterial and/or fungal contamination. The lid is then replaced and the contact plate is stored in an environment conducive to microorganism growth. A typical contact plate is pre-loaded with growth medium under sterile conditions and packaged for shipment to the end user.
Two known designs of contact plates are those that are the subject of U.S. Pat. Nos. 5,854,065 and 6,602,704. Both designs have the inherent drawback that the lid and base are held together by a compression fit that is often either too tight to allow ready disengagement between the lid and base or too loose, which can lead to accidental spillage or contamination when handling the contact plate. Another contact plate design is the subject of commonly assigned U.S. application Ser. No. 10/695,066, filed Oct. 27, 2003, now U.S. Pat. No. 6,969,606 which discloses a lockable contact plate wherein the locking members consist of pairs of radial sheaths and tabs.
What is needed therefore are a lockable Petri dish and a lockable contact plate that do not lock except upon application of a specific intentionally applied force, that provide a secure locking engagement between the cover and the dish, and which may be readily disengaged from the locking engagement. These needs are met by the present invention, which is summarized and described in detail below.